Article pubs.acs.org/acssensors
Antimony-Doped Tin Dioxide Gas Sensors Exhibiting High Stability in the Sensitivity to Humidity Changes Koichi Suematsu,*,†,# Miyuki Sasaki,‡ Nan Ma,‡ Masayoshi Yuasa,†,¶ and Kengo Shimanoe† †
Department of Energy and Material Sciences, Faculty of Engineering Science and ‡Department of Molecular and Material Science, Interdisciplinary Graduate School of Engineering Science, Kyushu University, Kasuga, Fukuoka 816-8580, Japan ABSTRACT: The type and amounts of oxygen adsorption species at various atmospheric humidity levels are important factors in improving the sensitivity to combustible gases and stability to humidity changes of SnO2-based resistive-type gas sensors. We investigated the effect of antimony (Sb) doping of SnO 2 nanoparticles on the stability of the sensitivity to humidity changes and oxygen adsorption species under humid atmosphere. No significant degradation of the sensitivity to hydrogen of Sb-SnO2 sensors was observed between 16 and 96 RH%, while an undoped SnO 2 sensor showed gradually decreasing responses with increasing humidity. An evaluation of oxygen adsorption species under humid atmosphere showed a transition from O2− to O− with increasing humidity from 16 to 96 RH%. However, the O2− adsorption sites were maintained on the surfaces of the SbSnO2, even as the humidity increased. Moreover, the extent of oxygen adsorption on the Sb-SnO2 was not obviously changed with increasing humidity. These results indicate that Sb atoms function as hydroxyl absorbers and also generate O2− adsorption sites in their vicinity. Additionally, Pd loading on the Sb-SnO2 further enhanced the sensor response under humid atmosphere, while maintaining the stability to humidity changes. Therefore, we successfully imparted stability to the sensitivity of SnO2 nanoparticles during humidity changes, representing an important improvement with applications to the development of high performance, practical, resistive-type gas sensors. KEYWORDS: SnO2, antimony, oxygen adsorption species, humidity, hydroxyl absorber
S
used to detect adsorbed oxygen on the SnO2 surface. The results have indicated that oxygen is dissociatively adsorbed on cationic Sn4+ sites. However, it has not yet been determined if the adsorbed species are O− (with one electron) or O2− (with two electrons). Recently, our group has focused on the effect of the oxygen adsorption species on the electric properties. According to theoretical calculations regarding oxygen adsorption, the relationship between the electric resistance and the oxygen partial pressure (PO2) can be expressed by the following equations in terms of the adsorbed oxygen species O− and O2−, respectively.17
urface interactions such as gas adsorption and reactions on semiconductor particles are of fundamental importance to electrochemical devices based on catalytic reactions. These interactions are especially vital to the properties of resistivetype semiconductor gas sensors, in which surface interactions are translated into electric signals. SnO2-based semiconductors are capable of both gas adsorption and combustion reactions, and so can act as high performance materials for sensors that detect combustible gases.1,2 Such semiconductors are thus the subject of fundamental studies and also research related to applications in various industrial fields and have been investigated with the aim of achieving high sensitivity3−7 and rapid response.5−8 In these devices, oxygen adsorption and surface reaction on the SnO2 change the electric resistance of the SnO2 particles, and this change acts as the gas detection signal; thus, the oxide functions as a transducer. This process relies on the dissociative adsorption of oxygen on the SnO2 surface with electron capture. For this reason, oxygen adsorption is the key affecting the performance of SnO2based resistive-type gas sensors. The relationship between oxygen adsorption on the SnO2 surface and electric properties such as electric resistance and sensor response has been insensitively studied. Various techniques such as Fourier transform infrared spectroscopy (FT-IR),9 O2-temperature-programmed desorption (TPD),10,11 electron paramagnetic resonance (EPR),12 electric conductivity measurements,13,14 and theoretical calculations15,16 have been © XXXX American Chemical Society
R 3 = ·(K O2·PO2)1/2 + c R0 a
(1)
⎧1 ⎫1/2 6ND R c = ⎨ c2 + ·(K O2·PO2)1/2 ⎬ + ⎭ R0 ⎩ 4 a 2
(2)
Here, R and R0 are the electric resistance in an atmosphere containing oxygen and in the absence of oxygen, respectively, a is the crystallite radius, ND is the donor density, KO2 is the oxygen adsorption equilibrium constant, and c is a constant. Received: May 15, 2016 Accepted: June 10, 2016
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DOI: 10.1021/acssensors.6b00323 ACS Sens. XXXX, XXX, XXX−XXX
ACS Sensors
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These equations indicate that the relative resistance (R/R0) is proportional to the square root of PO2 (in the case of O− adsorption) or the fourth root of PO2 (in the case of O2− adsorption). Based on experimental confirmation, both O− and O2− form competitively on the SnO2 surface, and the ratio of these species is influenced by temperature18 and the composition of the ambient atmosphere.19,20 In a dry atmosphere, O2− is the main adsorption species over 350 °C, while O2− and O− are competitively adsorbed at 300 °C.18 Water vapor causes the most harm to SnO2-based resistivetype gas sensors, because it adsorbs on the oxygen adsorption site and so interrupts oxygen adsorption.21,22 The associated attachment of hydroxyl groups occurs according to the reaction in the following equation.
(3) −
EXPERIMENTAL SECTION
Material Preparation and Characterization. Undoped SnO2 and Sb-doped SnO2 (Sb-SnO2) nanoparticles were prepared using a hydrothermal synthesis method, as reported previously.18 A mixed solution of SnCl4 and SbCl5 (0, 0.1, or 0.5 mol % Sb) was slowly added to a stirred NH4HCO3 solution in a dropwise manner. A stannic acid gel was obtained after allowing the mixture to stand for 12 h, and this gel was washed to remove the Cl− by centrifugation. The resulting gel was dispersed in deionized water and the pH of the dispersion was adjusted to 10.5 by adding aqueous NH3. The dispersion was then treated hydrothermally at 200 °C and 10 MPa for 2 h with stirring at 600 rpm to prepare either monodispersed undoped SnO2 or Sb-SnO2 solutions. Each solution was dried at 120 °C and subsequently calcined at 600 °C for 3 h to produce the undoped SnO2 and Sb-SnO2 nanoparticles. Pd-loaded and Sb-doped SnO2 (Pd/Sb-SnO2) nanoparticles was prepared by an impregnation method, using a procedure previously reported.19,30 Sb-SnO2 powder was dispersed in deionized water using ultrasonic irradiation, after which a Pd(NO3)2 solution was added to the dispersion. After drying the solution at 120 °C, the resulting powder was calcined at 500 °C for 3 h to produce the Pd/Sb-SnO2 nanoparticles. The crystal structures of the SnO2-based nanoparticles were analyzed using an X-ray diffractometer (XRD; RINT 2100, Rigaku) with Cu Kα radiation, and crystallite sizes were estimated from the resulting patterns based on Sherrer’s equation. The obtained nanoparticles were also observed by field emission scanning electron microscopy (FE-SEM; S-4800, Hitachi). Sb doping levels were determined using inductively coupled plasma atomic emission spectrometry (ICP-AES; performed at Nippon Steel and Sumikin Technology), and data were collected separately for the surface and total areas of the nanoparticles. For the analysis of particle surfaces, undoped SnO2 and Sb-SnO2 were etched with hydrochloric acid and the dissolved elements were quantified using ICP-AES. The nanoparticle interiors were assessed by completely dissolving the particles in a sodium peroxide solution, allowing the total Sb amounts to be determined using ICP-AES. Sensor Fabrication and Evaluation of the Electric Properties. In preparation for sensor evaluation, a Au comb-type electrode (180 μm line width, 90 μm distance between lines, 64 mm2 sensing area) was printed on an alumina substrate (9 × 13 × 0.38 mm3) and the sensor device was prepared using a screen printing method. During screen printing, the SnO2-based powders were mixed with α-terpineol to prepare pastes that were applied to the substrate using a screen printing method. The finished sensor devices were sintered at 580 °C for 3 h under a flow of synthetic air to remove the α-terpineol. To fabricate Pd/Sb-SnO2 sensors, the sintering temperature was changed to 480 °C. These devices were attached to a conventional gas flow apparatus equipped with a gas-mixing system and an electric furnace. For measurement of the sensor response to hydrogen, a test mixture consisting of 200 ppm hydrogen in synthetic air was sent to the quartz sensor chamber at a flow rate of 80 cm3/min using mass-flow controllers (SEC-series; HORIBA STEC). For the measurement of the relative resistance, various PO2 values diluted with nitrogen were sent to the chamber. Here, the PO2 was determined using an oxygen sensor based on calcium stabilized zirconia (CSZ). A humid atmosphere was prepared by the blowing the gases into deionized water and the resulting humidity was determined using a commercial humidity sensor (TR-77Ui; T&D Corporation). The devices were connected to a standard resistor under an applied voltage of 4 V DC to evaluate the electric resistance, and the electric signal of the device was measured using an electrometer (2701; Keithley Instruments). The sensor response (S) was defined as the ratio of the resistance in synthetic air (Ra) to that in air containing dilute H2 (Rg) (S = Ra/Rg). The relative resistance (R/R0) was defined as the ratio of the resistance in an oxygen-containing atmosphere (R) to that in nitrogen (RN2) (R/ R0 = R/RN2).
H 2O + O−(or O2 −) + 2Sn → 2(Sn − OH) + e− (or 2e−)
Article
2−
Here, Sn is a Sn ion on the SnO2 surface, O and O are the adsorbed oxygen species, Sn−OH is the adsorbed hydroxyl group at the Sn site, and e− is the SnO2 carrier electron released from the adsorbed oxygen. According to our previous results, the main oxygen adsorption species transitions from O2− to O− under humid conditions.19 Therefore, we theoretically proposed that hydroxyl groups preferentially adsorb on the O2− adsorption site.17 Hence, decreasing the quantity of adsorbed oxygen and changing the adsorbed species both significantly reduce the electric resistance and sensor response.17,19,23 For these reasons, it is important to protect the particle surfaces from hydroxyl poisoning, and surface modification by catalytic Pd loading on SnO2 particles is a useful means of doing so.24,25 Recently, we determined that Pd loading generates O2− adsorption site on the SnO2 surface and so increases resistance to hydroxyl poisoning.19 Other approaches to reducing the effect of humidity include particle surface modification with hydroxyl absorbers such as NiO,26 CuO,27 and Al,28 because hydroxyl absorbers preferentially capture a hydroxyl and provide an oxygen adsorption site. The main goal of research into these materials is to develop a highly sensitive sensor under humid atmospheres. The most important issue in this regard for practical gas sensors is the stability of the sensor response with humidity changes, because water vapor is essentially always present in the atmosphere at some level. Our own group has focused on the use of antimony (Sb) because of prior results showing that the effect of humidity on the electric resistance and response to NO2 of a Sb-SnO2 is improved relative to that of a SnO2.29 However, the mechanism by which this resistance to humidity degradation is obtained is not completely understood. Therefore, in the present study, we aimed to prepare Sb-doped SnO2 nanoparticles to inhibit sensor response degradation with humidity changes, and subsequently investigated the mechanism by which Sb-SnO2 reduces the effects of humidity, from the viewpoint of oxygen adsorption species. The effects of humidity on the sensor electric resistance in air and on the sensor response to hydrogen were assessed, and the relevant oxygen adsorption species were determined based on the PO2 dependence of the relative resistance in humid atmospheres. Moreover, to enhance the sensor response while maintaining good humidity tolerance, the effects of Pd loading on the Sb-SnO2 nanoparticles were studied. B
DOI: 10.1021/acssensors.6b00323 ACS Sens. XXXX, XXX, XXX−XXX
Article
ACS Sensors
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RESULTS AND DISCUSSION Materials Characterization. XRD patterns of the undoped SnO2, 0.1 mol % Sb-SnO2, 0.5 mol % Sb-SnO2, and 0.1 mol % Pd/Sb-SnO2 nanoparticles are shown in Figure 1. The obtained
Figure 1. XRD patterns of the (a) undoped SnO2, (b) 0.1 mol % Sb doped SnO2, (c) 0.5 mol % Sb doped SnO2, and (d) 0.1 mol % Pdloaded and Sb-doped SnO2 nanoparticles.
XRD patterns correspond well to the cassiterite structure of SnO2 (JCPDS: 41−1445), and impurity phases and peak shifts are not observed. The estimated average crystallite sizes of each of the SnO2 nanoparticles are summarized in Table 1. The
Figure 2. SEM images of the (a) undoped SnO2, (b) 0.1 mol % Sbdoped SnO2, (c) 0.5 mol % Sb-doped SnO2, and (d) 0.1 mol % Pdloaded and Sb-doped SnO2 nanoparticles.
air at 350 and 300 °C using undoped SnO2, 0.1 mol % SbSnO2, and 0.5 mol % Sb-SnO2 as a function of relative humidity at 25 °C are presented in Figure 3a and b, respectively. The
Table 1. Average Crystallite Sizes Estimated from XRD Patterns and Sb Contents Determined by ICP-AES Using Undoped SnO2, 0.1 mol % Sb-SnO2, 0.5 mol % Sb-SnO2, and 0.1 mol % Pd/Sb-SnO2 Nanoparticles sample undoped SnO2 0.1 mol % SbSnO2 0.5 mol % SbSnO2 0.1 mol % Pd/SbSnO2
avg crystallite size/nm
surface Sb content/mol %
total Sb content/ mol %
14 15
